If you want Squid to be part of a hierarchy of proxies or want Squid to fetch content from another proxy
cache_peer proxy.isp.com parent 8080 0 no-query no-digest ever_direct allow all
For Multiple parent
cache_peer proxy.isp1.com parent 8080 0 no-query no-digest default
cache_peer proxy.isp2.com parent 8080 0 no-query no-digest
Multiple parents with weight:
cache_peer proxy.isp1.com parent 8080 0 no-query no-digest weight=1
cache_peer proxy.isp2.com parent 8080 0 no-query no-digest weight=2
Multiple parents with round-robin:
cache_peer proxy.isp1.com parent 8080 round-robin no-query
cache_peer proxy.isp2.com parent 8080 round-robin no-query
cache_peer proxy.isp3.com parent 8080 round-robin no-query
In above examples proxy.isp1.com, proxy.isp2.com and proxy.isp2.com are other cache servers
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source:http://linuxpoison.blogspot.com/2007/10/13578175801149.html